"It shows content sharing has become a huge driving force online," said Matt Tatham, director of media relations at Hitwise. "People want information from friends they trust, versus the the anonymity of a search engine."

Facebook accounted for 7.07% of U.S. Web traffic that week, while Google (GOOG, Fortune 500) received 7.03%. The study compared only the domains Facebook.com and Google.com -- not, for example, Google-owned sites like Gmail.com.

Though the traffic levels were close, Facebook's year-over-year growth far outpaced Google's that week. The number of visitors to Facebook spiked 185% compared with the same period last year, while Google's traffic climbed just 9%.

"It's definitely a big moment for Facebook, even though they beat by a small margin," Tatham said. "We've seen it coming for quite a long time."

Facebook had never before beaten Google over a full weeklong period, though it has been the most visited site on recent holidays: Christmas Eve, Christmas Day and New Year's Day. Facebook was also the top site on the weekend of March 6-7.

But Tatham noted that when he added up traffic on all Google properties like Google Maps and YouTube, the company's sites comprised 11.03% of visits. Yahoo (YHOO, Fortune 500) was second with 10.98%.

Google.com had been the No. 1 site each week since Sept. 15, 2007, when ironically it passed another social networking site, MySpace.com, in order to take the crown.

Of course, the MySpace connection could be a bad omen for Facebook. MySpace enjoyed dominance on the social networking scene for years until it saw traffic plummet following Facebook's rise.

"By nature, the Web is ever-changing," Tatham said. "The Internet can be a fickle crowd."